Gas chromatography is an analytical procedure used for the analysis of components of a complex sample mixture of gases. Gas chromatography physically separates the components of the complex sample mixture by exploiting the different affinities of the components to a stationary phase. Typically, the sample mixture is injected as a plug into a mobile fluid (carrier gas) and forced to flow through a tube (separation column) containing a stationary phase. The stationary phase may be either a liquid or a solid. Since each component of the sample has a different affinity to the stationary phase relative to its counterparts, each component is retarded to a different extent by the stationary phase in its passage through the tube. As a result, the components of the sample mixture emerge from the tube at different times.
Upon exiting the tube, the concentration of the components of the sample mixture are detected by detectors that are positioned to receive the gas leaving the tube. The detectors must be able to respond to property differences between a zone of carrier gas and a zone of carrier gas mixed with a sample component. A commonly used type of detector is the thermal conductivity detector (TCD). This type of detector measures thermal conductivity, which is an indication of a substance's ability to conduct heat. By determining the thermal conductivities of the component/carrier gas mixtures that exit the tube, the TCD is able to determine the concentration and composition of the components of the sample mixture.
The TCD is comprised of a wire filament to which a voltage is applied to generate a current in the filament. As current flows through the filament, the filament becomes heated. The rate at which such heat is dissipated from the wire is determined by the surrounding environment that encompasses the filament. Thus, a zone of carrier gas surrounding the filament dissipates heat from the filament at a different rate than a zone of carrier gas mixed with sample components. The different rates of dissipation of heat or thermal conductivity are used to distinguish sample components. Specifically, the filament is held at a constant temperature and resistance, and the power required to maintain the filament at the constant temperature is measured. The power measurements indicate the different thermal conductivities of the components and therefore, identify the components of a sample mixture.
The analysis of the sample mixture is developed over the period of time it takes for all of the sample components to emerge from the column, and thus, the analysis of a continuous process stream is somewhat outdated by the time the analysis is completed. Further, additional time is lost in analysis because, in most instances, the chromatograph is housed in an analyzer shack site at some distance from the sample point. The sample is piped from the sample point to the analyzer shack site. Thus, the time it takes to transport the sample from the sample point to the chromatograph also delays the analysis.
Current field gas chromatograph instrumentation packages are quite large and require an abundance of plumbing and electrical connections. The number of connections and the size of the chromatograph contribute to the difficulty of installing, repairing and removing them. Moreover, one must have expertise in the areas of electricity, plumbing and chromatography analysis to properly perform such removal. Matters are complicated by the environment in which the packages are mounted. Generally, the packages are mounted in hazardous areas where adherence to strict safety and electrical standards is vital. In view of these complications, the unit is generally not removed when repairs are necessary; rather, the sometimes delicate and complex repairs are performed in the field at the analyzer chromatograph shack site. The completion of such repairs usually requires the chromatograph to be put in an "off-line" condition for an extended period of time in which the plant from which the samples are taken may have to either shut down or run blind without vital instrumentation.